1. Technical Field
The invention concerns the field of cell culture technology. It concerns production host cell lines containing vector constructs comprising a DHFR expression cassette. Those cell lines have improved growth characteristics in comparison to DHFR-deficient cell lines.
2. Background
The market for biopharmaceuticals for use in human therapy continues to grow at a high rate with over 900 biopharmaceuticals being evaluated in clinical studies and estimated sales of 50 billions in 2010. Over the years, an increasing number of biopharmaceuticals is produced from mammalian cells due to their ability to correctly process and modify human proteins. Successful and high yield production of biopharmaceuticals from mammalian cells is thus crucial and depends on the characteristics of the recombinant monoclonal cell line used in the process.
In biopharmaceutical production processes, yield is determined by two factors: the specific productivity (Pspec) of the host cell and the IVC, the integral of viable cells over time which produce the desired protein. This correlation is expressed by the following formula: Y=Pspec*IVC. Standard approaches to improve product yield therefore can be to increase either the production capacity of the host cell or viable cell densities in the bioreactor. One method to obtain higher IVCs is to improve the growth characteristics of cells, that means to generate cells which grow faster and to higher maximal cell densities.
The enzyme dihydrofolate reductase (DHFR) is one of the key enzymes of nucleotide synthesis. It catalyzes the reduction of di-hydrofolat to tetra-hydrofolat, a universal transmitter of C1-units in the synthesis of purin building blocks and of other metabolic pathways.
In biopharmaceutical industry and in academic research DHFR is widely used as a selection and amplification marker for the selective growth of stably transfected cell lines. Since DHFR is a non-dominant marker, this system is only applicable in cells which lack endogenous DHFR activity. Hence, when in the 1980's dhfr-negative Chinese hamster ovary (CHO) cells, CHO-DG44 and CHO-DUKX (B11), became available, they rapidly advanced to the host cell system of choice and are today the worldwide most frequently used mammalian production platform in the biopharmaceutical industry.